1. Field of the Invention
The present invention relates to an image processing method, an image processing apparatus and a computer readable recording medium recording an image processing program. Specifically, it relates to an image processing method for compressing image data, an image processing apparatus performing a process for compressing image data, and a computer readable recording medium recording an image processing program for causing a computer to execute a process for compressing image data.
2. Description of the Related Art
When image data is compressed in accordance with BTC (Block Truncation Coding) method, a balance between size of compression and image quality is determined depending on a block size and the number of representative colors.
When compressed data is subjected to editing process such as rotation, speed of processing significantly lowers if data size of image data after compression exceeds 32 byte border, up to which good data transfer efficiency is possible. The inventors disclosed, in Japanese Laid-Open Patent Publication No. 2008-092551, an image processing technique of adding, to an image, image processing information to be utilized in image processing. Using this technique, however, the data size of image data after compression exceeds 32 bytes, undesirably lowering the data transfer efficiency.
If the number of representative colors is reduced at the time of compression in order to reduce data size of the compressed data, however, image performance of an image, for example, including high-frequency component such as an opaque image, would be lost. If tone gradation is reduced, image quality degrades. As to the opaque image mentioned above, is it possible to set an object in an image to be semi-transparent by, for example, a word-processing software, and an opaque image refers to an image including such an object set to be semi-transparent.